Many commercial products are heat-sensitive and may lose efficacy or quality if they experience excessive ambient heat exposure before they are used. Examples of heat-sensitive commercial products include certain pharmaceuticals, medical products, and foodstuffs as well as some industrial products. Accordingly, time-temperature indicators have been provided that may monitor the cumulative ambient heat exposure of a host product and signal when a predetermined value that may correlate with a decline in the condition of the host product has been reached. The signal may be a color change, for example a darkening of an indicator area, and may be generated by a heat-sensing agent such as a diacetylenic compound, or another technology, that integrates the heat exposure, as measured by temperature, over time. Some examples of heat-sensitive diacetylenic compounds, and of time-temperature indicators employing them, are described in U.S. Pat. No. 8,067,483 to Prusik et al.; U.S. Patent Application Publication No. 2009/0131718 by Baughman et al.; and U.S. Patent Application Publication No. 2011/0086995 by Castillo Martinez et al. Other patents and patent publications describing various time-temperature indicator technologies are cited elsewhere herein.
Some host products are also sensitive to short-lived peaks, or spikes, of heat exposure that may not have sufficient cumulative heat value to cause an accompanying time-temperature indicator to signal that a heat exposure limit may have been reached. Some examples of such products are vaccines and other medical products which include a proteinaceous active ingredient.
Accordingly, there is a need for a dual-function heat indicator that may effectively monitor cumulative ambient heat exposure and peak ambient heat exposure and provide a clear signal of possible excess heat exposure.
Various proposals are known for indicators that may signal past exposure to temperatures exceeding a threshold. For example, U.S. Pat. No. 5,709,472, and its divisional patent, U.S. Pat. No. 6,042,264, both to Prusik et al., describe and claim a time-temperature indicator label for measuring the length of time to which a product has been exposed to a temperature above a pre-determined temperature. Also, U.S. Pat. No. 7,517,146 to Smith et al. describes an excess temperature indicator that may provide a visual indication of past exposure of perishable, maturing and other host products to an elevated temperature exceeding a threshold temperature.
Further, U.S. Pat. No. 5,057,434 to Prusik et al. (“Prusik et al. '434” herein) relates to an improved time-temperature indicator device useful in monitoring the environmental exposure of products that undergo progressive quality changes in response to such exposures. See, e.g., column 1, lines 5-8 of Prusik et al. '434. As described, a cumulative time-temperature indicator and a threshold indicator may be integrated into a single device. Further, the device may gradually and irreversibly develop color as a function of time and temperature and more closely monitor the actual condition of a deteriorative product than does a single indicator, See, e.g., abstract of Prusik '434. The capabilities of the system may be enhanced by a barrier layer that delays the color development action. See, e.g., column 9, lines 25-33.